Bullseyes are common with the Inverse Distance to a Power gridding method. To remove the bullseye effect, regrid the data and change the Smoothing value to something other than 0 on the Grid Data - Inverse Distance to a Power - Options dialog page. The smoothing factor parameter allows you to incorporate an "uncertainty" factor associated with your input data. The larger the smoothing factor parameter, the less overwhelming influence any particular observation has in computing a neighboring grid node. For example, instead of 0, you could try a smoothing value of 10. If that is too smooth, you can step back the smoothing value to 5 to see if that is better.
If you use the Kriging gridding method, specifying a nugget effect will likely smooth things out. The nugget effect is made up of two components: Nugget Effect = Error variance + Micro Variance. To add a nugget effect, click to the Grid Data - Kriging - Variogram page of the dialog. In the Nugget Effect section, enter the Error Variance and Micro Variance, and click OK. When testing the nugget effect, set the micro variance to zero and try some different values for the error variance to judge the effect.
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| Inverse Distance to a Power - Smooth parameter | Kriging - Nugget Effect parameters |
If the above suggestions don't help, you could try:
- A different gridding method (ie. Minimum Curvature or Triangulation with Linear Interpolation).
- Modifying the search radius on the Options page of the Grid Data dialog.
- Including "dummy data" in your original data file to help Surfer determine contour locations.
- Filtering the grid file (Grids | Edit | Filter). You may want to use a larger "neighborhood" in the filter than the typical 3x3 used in most pre-defined filters. For example, you could try a Gaussian low-pass filter with a 9x9 neighborhood and run it for 3 passes. This may certainly smooth things out, but you may not like the results. You may need to experiment with some different settings to see what you like.
- Editing the grid in the Grid Editor window by clicking Grids | Editor | Grid Editor, selecting the grid and clicking Open. Edit the Z values of the grid nodes themselves using the Select tool or use the Push up or Pull down tools until the bull's eye effect is removed. Then save the grid by clicking File | Save (or Save As to save it to a new name) and click the X on the view tab to close it. Then update the map with the File | Reload Map Data command.
Updated October 2021
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